Abstract
Recently, MoO3, which is typically used as an anode buffer layer in organic photovoltaic cells (OPVCs), has also been used as a cathode buffer layer (CBL). Here, we check its efficiency as a CBL using a planar heterojunction based on the CuPc/C60 couple. The CBL is a bi-layer tris-(8-hydroxyquinoline) aluminum (Alq3)/MoO3. We show that the OPVC with MoO3 in its CBL almost immediately exhibits lower efficiency than those using Alq3 alone. Nevertheless, the OPVCs increase their efficiency during the first five to six days of air exposure. We explain this evolution of the efficiency of the OPVCs over time through the variation in the MoO3 work function due to air contamination. By comparison to a classical OPVC using a CBL containing only Alq3, if it is found that the initial efficiency of the latter is higher, this result is no longer the same after one week of exposure to ambient air. Indeed, this result is due to the fact that the lifetime of the cells is significantly increased by the presence of MoO3 in the CBL.
Highlights
Recent studies show that if current progress is continued, organic photovoltaic cells (OPVCs) will have a unique advantage for large scale power generation [1]
These values measured in situ by ultraviolet photoelectron spectroscopy (UPS) after deposition in ultra-high vacuum are ionization potential (IP) = 9.7 eV, χ = 6.7 eV and Wf = 6.9 eV
After measuring the valence band maximum energy of MoOx by UPS to be equal to 7.1 eV and the band gap of MoO3 to be 3 eV, they conclude that the conduction band minimum is at approximately 4.1 eV, which is in good agreement with the work function of Al (4.3 eV)
Summary
Recent studies show that if current progress is continued, organic photovoltaic cells (OPVCs) will have a unique advantage for large scale power generation [1]. It is well accepted that its band gap is approximately 3.1 eV and that it is an n-type material, yet the values of its ionization potential (IP), its electronic affinity (χ) , and its Fermi level (Wf) are still under discussion These values measured in situ by ultraviolet photoelectron spectroscopy (UPS) after deposition in ultra-high vacuum are IP = 9.7 eV, χ = 6.7 eV and Wf = 6.9 eV. After measuring the valence band maximum energy of MoOx by UPS to be equal to 7.1 eV and the band gap of MoO3 to be 3 eV, they conclude that the conduction band minimum is at approximately 4.1 eV, which is in good agreement with the work function of Al (4.3 eV) This energy alignment results in a relatively low electron injection barrier height. We show that the lifetime of the OPVCs with MoO3 in their CBL is significantly improved
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